1. Field of the Invention
The present invention relates to the RC-FSK (Return to Carrier Frequency Shifting Keying) modulation system and the decoding technique thereof, and more particularly to a method and an apparatus for adaptive clock reconstruction and a method and an apparatus for decoding in audio frequency communication.
2. Related Art
In communication system, modulation is a process of implanting information to a carrier signal and the information is combined into the carrier signal to transfer the original information to a electrical signal adapted for transmission. Generally speaking, modulation is used for wireless broadcast, wireless communication or digital communication on telephone line and so on. Depending on the difference of the modulated signals, the modulations can be sorted by digital modulation (FSK, ASK, PSK) and analog modulation (FM, AM, PM). Those different modulations are techniques utilizing different methods to combine information with carrier. The reverse process of modulation calls demodulation which is used for restoring the original information.
Modem (modulation and demodulation) technology in audio frequency communication is used in recent years. It mainly utilizes the audio frequency carrier to transmit digital information by TV, radio, audio, and so widely used the traditional audio equipment.
The modem technology about multi-carrier with QPSK (Quadrature Phase Shift Keying) in demodulation in audio frequency is depicted in U.S. Patent Publication No. 20030212549A1 and U.S. Patent Publication No. 20040001553A1. The abovementioned modem technology can achieve transmission with high bit rate. Nevertheless, the abovementioned modem technology has to use multiple bands. Because of the transmission energy is scattered in multiple carrier bands, the multi-carrier communication would have poor anti jamming capability on the daily existence of the wideband noise (such as voice, TV broadcast, hair dryer sound).
FSK (Frequency-Shift Keying) is a modulation technique by which data is transmitted by the modulated signal changing in the frequency. The most common case is binary FSK. Binary FSK is to utilize two different frequencies to respectively represent two different binary symbols which respectively represent logic 0 and logic 1. Since FSK belongs to a sort of single-carrier modulation, the energy is highly concentrated in a single carrier, and the environment wideband noise have less probability to match the communication frequency exactly, so single-carrier FSK is more suitable for audio frequency communication.
FIG. 1A is a circuit block diagram depicting a conventional FSK demodulation circuit 10. Referring to FIG. 1A, the FSK demodulation circuit 10 includes an analog-to-digital converter (ADC) 101, a spectrum analyzer 102, a binary data decision circuit 103, a package header comparing circuit 104 and ECC (error checking and correction) circuit 105.
The ADC 101 receives an analog modulated signal AS and converts the analog modulated signal AS to a digital signal DS. The spectrum analyzer 102 receives the digital signal DS and performs the spectrum energy analysis to the digital signal DS so as to output a frequency envelope FE. The binary data decision circuit 103 receives the frequency envelope FE and a symbol clock signal CK and samples the frequency envelope FE according to the symbol clock signal CK to decide a symbol binary data BD. The package header comparing circuit 104 is used for comparing the binary data BD with preset package header bits to obtain a package message PGM. The ECC circuit 105 receives the package message PGM to perform error correction to ensure the correctness of the package message PGM.
FIG. 1B is a waveform diagram depicting a conventional FSK demodulation. Referring to FIG. 1B, the waveform 106 represents a carrier; the waveform 107 represents a modulated signal; the waveform 108 represents a binary data to be transmitted; the waveform 108 represents the abovementioned clock signal CK. Since the binary data decision circuit 103 requires the clock signal CK to determine the transmitted data 108 in positive level of the clock signal CK, clock synchronization between the transmitter and the receiver in advance are required for the abovementioned modem technology in audio frequency, and then the transmitter can start to transmit the valid data. If a deviation of clock signal or an interference of the clock signal occurs in one of transmitter and receiver, timing error will be occurred at the transmitter or the receiver to induce error in the decoded data. Moreover, in process of transmission, the error of the clock signal of both of the transmitter and the receiver will be accumulated so that transmission data rate and robustness of the transmission system are hardly increased in the conventional modem technology in audio frequency.
U.S. Patent Publication No. 20050254344A1 and U.S. Pat. No. 4,320,387 relates to the single-carrier modulation in prior art. The U.S. Patent Publication No. 20050254344A1 is about FM or AM to be used for the modem technology thereof, and the U.S. Pat. No. 4,320,387 is about the FSK to be used for modem technology thereof. Nevertheless, in the abovementioned patent disclosures, since the frequency and the phase of the clock signal of the transmitter should be obtained by the receiver for performing demodulation, the clock synchronization or handshaking should be performed before transmission to confirm the frequency and the phase of the clock signal between the transmitter and the receiver in advance.